Treating austenitic stainless steels



United States Patent U.S. Cl. 14812.4 Claims This invention relates to austenitic stainless steels. More particularly, the invention concerns improving the resistance of austenitic stainless steels to stress corrosion cracking.

All stainless steels are iron-chomium alloys and may be grouped into three classes-martensitic, 'ferritic and austenitic. Generally, austenitic stainless steels are alloys of iron, chromium and nickel, with or without other elements. There are also austenitic stainless steels in which nickel is partially or wholly replaced by manganese and nitrogen.

Austenitic stainless steels have had wide-spread use as construction materials in nuclear reactors and in the chemical industry. However, it has been found that under certain adverse conditions such as exposure to chloride environments, austenitic stainless steels are susceptible to stress corrosion cracking. Because of the hazards associated with the failure of critical components in nuclear reactors as well as the expense involved in replacing components that have failed in the chemical industry, austenitic stainless steels with improved resistance to stress corrosion cracking are in demand.

The present invention concerns improving the resistance of austenitic stainless steels, with or without nickel, to stress corrosion cracking without sacrificing physical properties.

According to the invention, several different methods are provided for increasing the resistance to stress corrosion. Among the austenitic stainless steels with which the invention may be practiced are three of the most common types having AISI designations 301, 304 or 304L, 308 or 309, 310, and also partially or wholly nickel-free austenitic stainless steels. The first types of steel contain about 18% chromium, 8% nickel. The second type contains about 20% to 24% chromium, 12% nickel, and the third contains about 25% chromium, 20% nickel. Austenitic stainless steels in which nickel is partially replaced by manganese and nitrogen (AISI Type 201 and 202 stainless steels) contain about 18% chromium, 4% nickel, 7.5% manganese and 0.25% nitrogen. Nickel free austenitic stainless steels contain about 18% chromium, 15% manganese and 0.50% nitrogen. Each of these steels may also contain small amounts of certain other elements added to enhance various properties or to immunize the steels against the action of certain harmful impurities and, in that event, are referred to by other AISI designations. Thus, the invention is applicable to related types of austenitic stainless steels as well as those referred to above.

The several processes within the purview of the invention are as follows:

(1) Low-temperature hot working of austenitic stainless steel followed by annealing at low optimum temperatures.

(2) Cold working either conventionally hot worked austenitic stainless steel or low-temperature hot worked steel followed by annealing at low optimum temperatures.

(3) Low temperature hot working followed by (with or without intermediate annealing) cold working and then conventional annealing of austenitic stainless steel.

(4) Conventional annealing of austenitic stainless steel followed by refrigeration at low temperatures to obtain 3,446,333 Patented May 27, 1969 martensite and annealing to revert martensite to austenite; and

(5) Conventional annealing of austenitic stainless steel, cold working to obtain martensite, heating and annealing to revert martensite to austenite, refrigeration at a low temperature to obtain matensite and reannealing to revert martensite to austenite.

The improvement in resistance to stress corrosion cracking obtained by practicing each of the above methods varies to some extent, but in every case represents at least 10-fold improvement. In some cases, the improvement may be as much as a IOU-fold increase in the resistance to stress corrosion cracking.

Each of the several methods described above has some particular advantage which may affect its selection over the others in various situations. Method I is useful when high mechanical properites as well as high stress corrosion resistance is desired. Method II has the advantage that no special control of hot rolling is needed. Method III enables the use of conventional mill facilities and mill practices with optimum stress corrosion resistance obtained by use of the low-temperature finishing operation. Methods IV and V are practiced on metastable austenitic stainless steels and have the advantage that stress corrosion resistance can be obtained on steel products in an extremely deformed condition.

The methods referred to above will be individually described below. For comparison, conventional ingots of austenitic stainless steel were heated to hot working temperatures between 2100 F. and 2400 F., depending on composition, and then forged and/or hot rolled to semifinished mill products such as blooms, billets and slabs. After conditioning, the semi-finished products are reheated and then hot rolled to finished mill products. The blooms or billets are customarily hot rolled to bars and rods, and the slabs are processed in a hot strip mill for the production of sheets or plates. The finished mill products are then annealed at a temperature between about 1900 F. and 2100 F. (usually at about 2000 F.) and rapidly cooled. Finishing operations, such as conditioning and cold reduction, are then performed.

Method I Contrary to the well-established practice, described above, we have discovered a marked improvement in resistance of austenitic stainless steels to stress corrosion cracking may be effected by lowering the conventional hot rolling finishing temperature and subsequent annealing temperatures. Moreover, the improvement obtained is achieved without sacrificing any of the critical mechanical properties such as formability and strength. In fact, it has been found that the yield strength is critically improved over those obtained with conventional processing. These advantageous results are obtained by hot working austenitic stainless steel ingots to a finishing temperature in the range of from about 1200 F. to about 1800 P1, preferably about 1200 F. to about 1700" F., and subsequently annealing at temperatures between 1000 F. and 1600" F. followed by quenching. By hot working to the desirable low finishing temperature is meant that the hot working step is performed so that the last deformation is completed within the desired temperature range. Ordinarily, ingots are heated to star-ting temperatures higher than the finishing temperatures and cool during hot working. In the conventional practice, the hot rolled finishing temperature is above 1800 F.

As an illustration of the improved results obtained by practicing this method in accordance with the invention, two steels of AISI Type 304 and 304L were processed having the composition described in Table I.

TABLE I Composition, percent Steel (3 N Ni Cr Mn P s Si Cu Mo Type 304L.... 0.020 0. 024 10.7 13.3 1.23 0. 019 0.021 0. 0. 054 0.15 T e 304 0. 033 0.025 9.05 13.3 1. 31 0.022 0.010 0. 43 0. 035 0.23

The steel specimens were l-inch thick plates which 304 stainless steel cracked in an average of only hours were hot rolled to 0.50-inch thickness in five passes of at a stress level of 22,500 p.s.i., whereas the same steel 0.1-inch reduction each. Both steels were rolled with a processed according to the invention lasted for an average starting temperature of 2250 F. The finishing temperaof 255 hours before failure even when tested at the ture for the Type 304L was 1600" F. and for the Type higher stress level of 41,000 p.s.i. 304 was 1405 F. Other specimen blanks (0.5 X 0.5 X 3.0- Method H inches) were cut from the hot rolledv plates and tension- YP Speclmens (0-252'111Ch dlametel') were made from Conventionally, after hot rolling and mill annealing,

the blanks The P 304 and 2 Specimens were austenitic stainless steels are conditioned and further renealed at 1200 Q respecnvely for duced in thickness by a cold reduction process followed hour followed by quenching 1n ice water. The mechaniby annealing at about 20000 and rapid Cooling Wa cal Propflties fe from above :eatments are shown 9 have found, contrary to the well-established practice, that m Table Comparison of Processing y Conventlonal a marked improvement with respect to resistance of practice is also shown in Table II. As is se n, the Steels austenitic stainless steels to stress corrosion cracking may processed according to our invention show greatly inbe effected by annealing the cold worked steels at lower creased yield strengths with other mechanical properties than normal temperatures. Conventional annealing is perbeing substantially unchanged. formed at temperatures between 1900 F. and 2100" F.

TABLE 11 Yield strength, p.s.i. Tensile strength, psi. Reduction in area, percent Steel Conv. proc. Impr. proc. Conv. pr 00. Impr. proc. Conv. proc. Impr. proc. T e 304L 34, 000 51,000 82,000 87,000 80.0 78.0 T e 304 30,000 55,000 85,000 90.000 81.0 70.0

The superiority of the steels processed according to this To illustrate the results obtained with this method of method of the invention with respect to resistance to 3 the invention, two BOO-pound ingots of the composition stress corrosion cracking is shown in Table III where described in Table IV were heated to a hot working temthis property is compared with that of austenitic steels of perature of 2150 F. The ingots were hot rolled into the same composition processed according to convention- 0.5-inch thick plate to a conventional finishing temperaal practice. The steels are tested for stress corrosion pro ture of between 1800 F. and 1900 F. and subsequently erties in the customary manner by exposure to boiling 4O cooled in air. Specimen blanks (0.5 x 0.5 x 3.0-inches) magnesium chloride, 3. common testing medium, at a were cut from the plates and were annealed at 1950 stress level of 75% of the actual yield strength of the F. for minutes followed by quenching in water. From test specimen. the annealed blanks, tension-type specimens (0.252-inch diameter) were then machined. Some of these specimens TABLE 111 were cold worked by elongating them about 30%, after Stress Average time 0 o level, to failure, whlch the specimens were annealed at 1400 F. for 1 Math Pmcessmg hours hour followed by quenching in ice water. Mechanical Conventional 25,000 10 properties, with or without the annealing treatment, are

D Improved 38,300 2458 d d T V It b d h t h 1 Type 304 Conventional 22,500 5 ese11 e in a e can e note t a t e stees Improved 41,000 255 processed according to the lnvention show higher yield 1 Aver-age omuee Specimens, and tensile strengths than those processed according to 2 Average spemmensthe conventional practice.

TABLE IV Composition, percent Steel 0 N Ni Cr Mn P s 31 TABLE v Yield strength, p.s.i. Tensile strength, p.s.i. Reduction in area, percent Steel Conv. proc. Impr. proc. Conv. proc. Impr. proc. Conv. proc. Impr. proc.

A 23,000 31,000 72,000 80,000 78. 4 77. 7 B 30,000 70,000 as, 000 107,000 75. 0 01. 1

The conventionally processed Type 304L cracked in The superiority of the steels processed according to the this solution in an average of 10 hours at a stress level invention with respect to their resistance to stress corof 25,600 p.s.i. In contrast, steel processed according to rosion cracking are shown in Table VIQConventionally the invention lasted for an average of 458 hours before processed steel A cracked in a boiling magnesium chlocracking even when tested at the higher stress level of ride solution within an average of 39 hours at a stress 38,300 psi. Similarly, conventionally processed Type level of 21,000 psi. The same steel processed according to the invention did not fail with an average of 555 hours when tested at a stress level of 23,000 p.s.i. under identical conditions. The steel B cracked in a boiling magnesium chloride solution within an average of 67 hours at a stress level of 29,000 p.s.i., whereas the same steel The superiority of the steel processed according to our invention with respect to its resistance to stress corrosion cracking is shown in Table VIII, wherein the conventionally processed Type 304L stainless steel is compared with a similar steel processed in accordance with the processed according to the invention did not fail within invention.

TABLE VIII Average time to Method of failure, Steel processing Treatment hours C Improved Hot rolled; cold-reduced (50%); an- 11,000

nealed at 2,000 F., 1 minute; air cooled C .do Hot rolled; annealed at l,950 F., 750

minutes; W.Q.; cold reduced (50%); annealed at 2,000 F., 1 min. or 5 min.; air cooled. Commercial Conventional Mill annealed 2 Type 304L.

1 No failure at indicated times.

515 hours, although it was tested at a stress level more The conventionally processed steel Type 304L cracked than twice (59,000 psi.) that used for the conventionin the boiling magnesium chloride solution within an ally processed specimens. average of 2 hours, whereas the steel processed in ac- TABLE VI Co-rdance with the invention did not crack within 1000 hours in the same test. Also, introducing an intermediate stress Avert ige f ug annealing treatment between the hot working and the Steel Method Processing 52 cold working operations does not eliminate the improve it fg z g a1 g ggg 252g n tained by the application of this method. B: c0vgirii6fiaiIIII 203000 01 Method rv B.-. Improved 59,000 515 Average of four specimens. This method and Method V described below are par- 2N0 iailure at indicated times. ticularly well suited to metastable austenitic stainless d In steels. Typical of these steels are AISI Types 301, 304 Metho and 304L. Because of the metastability, a special practice The advantage of this method of the invention is that is observed to increase the martensite development for it can employ conventional annealing temperatures and availability in reverting to austenite. special annealing practices need not be observed. This Contrary to the well-established conventional practices, method comprises low temperature hot working followed it has been discovered that in these metastable steels, by heavy cold reduction and subsequent annealing at conresistance to stress corrosion cracking may be achieved ventional temperatures, or hot working at low temperaby refrigerating annealed austentic stainless steels at very f fl d b annealing d 1d d ti ith a blow temperatures to obtain a substantially martensitic sequent fi l conventional anneaL structure, and then reverting to martensite formed to aus- To illustrate the improved properties of steel processed tenite y nn aling at conventional temperatures, usually i accordance i h hi h d of h i ti a 300- about 2000 F. or at a lower temperature just above the pound vacuum melted ingot f a steel f iti C, temperature at which martensite transforms to austenite. shown in Table VII, was heated to 2150 F. for 1 /3 The Superior properties of the product produced in hours and then hot rolled to 0.755-inch thick plate in eight accordance wi h t i me of the i v n n is shown y passes to a finishing temperature of about 1675 F. prior the ll wing example. Steels D and E of chemical comto air cooling. The plate was then further hot rolled Position Shown in Table IX, which are similar to AISI from 0.755-inch plate to 0.1l7-inch thick sheet. The Type 301 stainless steel, were prepared in 300-pound starting temperature as well as the number of hot rolling Vacuum, induction-melted ingots- The ingot Of Steel C0111- passes were the same as in the first hot rolling operation, position D was hot rolled to 0.50-inch thick plate. Some but the finishing temperature was between 1500 F. of these hot rolled plates were annealed at 1950 F. and 1600 F. After air cooling, the sheet was then cold for about 45 minutes and then quenched in Waterreduced to 0.050-inch thick, annealed at 2000" F. for 1 Specimen blanks 0.5 x 0.5 X 0.3-inches were cut from the minute i a lt b h, d l d i i Th h iwl hot rolled plates as well as from the hot rolled plus compositions of this steel as well as the commercial Type annealed plates and 0.252-inch diameter tension speci- 304L are given in Table VII. mens were made from the blanks. Some were given the TABLE VII Composition, percent Steel 0 N Ni Cr Mn P 5 si 0 0. 019 o. 035 7. 9 17. 9 1. 50 0.027 0. 015 o. 53 Type 304L 0. 02 0.026 9. 4 19. 6 1. 2s 0. 022 0. 017 o. 61

Olsen cup specimens were used to determine the effect double phase transformation treatment described preof deformation on the stress corrosion properties of the viously. In this method, the hot rolled steels were ansheet. This involves indenting a 3% x4-inch sheet with a nealed at about 1950 F. or a lower temperature of Ma-inch diameter ball to of the extension required 1400 F., and the annealed specimens were refrigerated to fracture the sheet. The Olsen cup specimens were in liquid nitrogen at -320 F. to obtain martensite. then exposed to a boiling 42% magnesium chloride 75 The martensite was then reverted to austenite by heat solution.

treating at 1950" F. or 1400 F. As may be seen from 7 Table X, this cycle was in some experiments repeated twice. It should be understood however that the cycle may be repeated as many times as required to achieve the desired properties.

austenite has been found to be at about 1200 F. for these representative compositions. The minimum temperature for reversion does vary to some degree with the composition of the steel and a substantial grain can be TABLE IX Chemical composition, percent Steel N Ni Cr Mn 1? S Si 818%; 318%? $13 512 i123 3:82? 818%? 8:23,

The superior resistance to stress corrosion cracking of the specimens processed according to this method of the invention is shown in Table X. In this table, a comparison of the properties of specimens made from the same steel but processed by conventional practice and with the properties of specimens processed in accordance with this method of the invention are shown. After heat treating, the specimens were prcstrained 10-30%, stressed to 75 or 100% of the load required to obtain the prestrain, and exposed to a boiling 42% magnesium chloride solution. The specific heat treatments, amount of prestrain, and the stress corrosion results are given in Table X.

Method V Austenitic stainless steels which are diflicult to transform to martensite by the refrigeration treatment in Method TV can be improved by practicing Method V. Among these steels would be those with M temperatures TABLE X Stress level Time to Amount of Percent of failure, hr. prestraln, load for Pounds per Method of processlng percent prestrain square inch 1 2 Conventional HR+A (l,950 F.) 10 75 42, 800 Improved. HR+A (1,950 F.)+R+A (2,000 F.) 10 75 51, 300 Conventro HR-l-A (l,950 F. 15 75 63, 700 Improved- HR+A (1,950 F.)+R+A (2,000 F.). 15 75 52,900 Do HR+A (1,400 F.)+R+A (1,400 F.)+ 15 100 127, 100

R+A (1,400 F.). Conventional HR+A (1,950 F.) 20 75 69, 300 Improved HR-I-A (1,950" F.) +R+A (2,000 F.) 20 75 83, 600 Conventional HR+A (1,950 F.) 75 100, 000 Improved. HR+A (1,400 F.)+R+A (l,400 F.)+ 30 75 119, 600

R+A (1,400 F.).

1 No failure at indicated times. HR Hot rolled; A-Annealed; R Refrigerated in liquid nitrogen (320 F.).

TABLE XI Stress level Amount of Percent of Time to failure prestrain, load for 7 Pounds per for one Method of processing percent prestrain square inch specimen, hr. Conventional HR+A (1,950 F.) 5 75 36, 500 4 Improved H+A20(1,200 F.)+R+A (1,200 F.)+R+ 5 75 54, 000 118 Do HR+D (20%)+A (1,200 F.)+R+A (1,200 5 75 66, 200 218 F.)+R+A (1,20 Do HR-l-D (20%) at 320 F.+A (1,200" F.)+ 5 75 92,200 1 571 R+A (1,200 F.)+R+A (1,200 F.). Do HR+A (2,000 F.)+R repeated four times 5 75 29,800 6 +A ,000 F. Do HR+A (1,200 F.)+R repeated four times 5 75 55,900 195 +13. (1,200 F. Do HR-l-D (20%) at 320 F.+A (1,200 F.)+ 17 75 128, 300 644 R-i-A (1,200 F.)+R+A (1,200 F.).

1 No failure at indicated times.

HR Hot rolled; AAnnealed; R Refrigerated in liquid nitrogen (320 F.); D-Cold worked by elongation.

As can be seen, at least a 100-fold improvement in stress corrosion resistance was obtained in the specimens processed in accordance with the invention. The results also show that the degree of improvement depends on the annealing temperature as well as the reversion temperature used. For example, as shown in Table XI, when hot rolled steel is annealed at 2000 F. and refrigerated with the cycle being repeated four times before final annealing, the specimen failed in 6 hours. In contrast, when the same hot rolled steel was annealed and refrigerated with the cycle being repeated four times before final annealing at 1200 F., the specimen failed in 195 hours. It should further be noted that this improvement is achieved even though this specimen was tested at almost double the stress level of the other specimen. The optimum temperature for annealing of the hot rolled substantially below room temperature, e.g. F. or lower. Steels with higher M temperatures, e.g. -30 F., can be treated successfully according to Method IV.

In this method according to the invention, hot rolled steels are annealed and then cold reduced either at room temperature or preferably at a temperature as low as that of liquid nitrogen (320 F.). In this manner, the austenite can be substantially transformed to martensite. Martensite is then reverted to austenite by heat treating at least at the A; temperature, i.e. the temperature at which martensite is reverted to austenite. This is indicated by a lack of any magnetic activity, as can be determined by a hand magnet. Following reversion to austenite, Method 1V is then applied and the steel is refrigerated at very low temperature to obtain a substantially martensitic structure and then exposed to elevated temperatures to specimens as well as the reversion of martensite to 75 revert martensite to austenite.

As an example of this method according to the invention, steels of composition E (see Table IX) were prepared in 300-pound vacuum induction melted ingots and hot rolled to 0.5-inch thick plates. Some of these plates were'annealed at 1950 F. for about 45 minutes and quenched in water. Specimen blanks 0.5 X 0.5 X 3.0-inches were cut from hot rolled plus annealed plates and tension specimens were made from the blanks. Some specimens were given the double phase transformation described above, i.e. refrigeration followed by annealing above the A; temperature, preferably at about 1200 F.

The superior resistance to stress corrosion cracking of the steel processed in accordance with this method of the invention is shown in Table XI above, wherein these results are compared with those obtained for specimens made from the same steel but processed by the conventional practice. After heat treating, the specimens were prestrained, stressed to 75% and exposed to a boiling 42% magnesium chloride solution. The specific heat treatments, amounts of prestrain and stress level and stress corrosion results are given in Table XI. More than a IOU-fold improvement was obtained on the specimens processed according to this method of the invention.

It is apparent from the above that various changes may be made without departing from the invention. Accordingly, the scope of the invention should be limited only by the appended claims.

We claim:

1. A method of improving the resistance of austenitic stainless steels to stress corrosion cracking which comprises hot working said steel to a finishing temperature in the range of from about 1200 F. to about 1800 F., low temperature annealing in the range of from about 1000 F. to about 1600 F. and thereafter quenching.

2. A method according to claim 1 wherein said low temperature annealing is performed at about 1200 F.

3. A method according to claim 1 wherein said low temperature annealing is performed at about 1400 F.

4. A method of improving the resistance of austenitic stainless steels to stress corrosion cracking which com- 40 prises cold working austenitic stainless steel which has been hot worked, annealing said cold worked steel in the temperature range of from about 1200 F. to about 1800 F. and thereafter quenching.

5. A method of improving the resistance of austenitic stainless steel to stress corrosion cracking which comprises hot working said steel to a finishing temperature in the range of from about 1200 F. to about 1800 F., cold working said steel and annealing said cold worked steel at temperature in the range of from about 1900 F. to about 2100 F.

6. A method according to claim 5 wherein said steel is hot worked to a finishing temperature in the range of from about 1200 F. to about 1700 F.

7. A method of improving the resistance of metastable austenitic stainless steel to stress corrosion cracking, which comprises annealing said steel, refrigerating said steel at a low temperature sufficient to obtain a substantially martensitic structure and annealing at temperature in the range of from about 1200 F. to about 2000 F. to revert martensite to austenite.

8. A method according to claim 7 wherein said steel is refrigerated and annealed more than once.

9. A method of improving the resistance of metastable austenitic stainless steel to stress corrosion cracking which comprises annealing said steel, cold working said steel until the steel becomes strongly magnetic, heating said steel to revert martensite to austenite and annealing same, refrigerating said steel at a low temperature sufiicient to obtain a substantially martensitic structure and annealing at a temperature above the A temperature to revert martensite to austenite.

10. A method according to claim 9 wherein said annealing above the A temperature is performed at about 1200 F.

References Cited UNITED STATES PATENTS 2,820,708 1/1958 WaXweiler 148-12.3 2,363,736 11/1944 Lynn 148-12.4 2,266,952 12/1941 Bloom 14812.4

L. DEWAYNE RUTLEDGE, Primary Examiner. W. W. STALLARD, Assistant Examiner. 

1. A METHOD OF IMPROVING THE RESISTANCE OF AUSTENITIC STAINLESS STEELS OF STRESS CORROSION CRACKING WHICH COMPRISES HOT WORKING SAID STEEL TO A FINISHING TEMPERATURE IN THE RANGE OF FROM ABOUT 1200* F. TO ABOUT 1800* F., LOW TEMPERATURE ANNEALING IN THE RANGE OF FROM ABOUT 1000* F. TO ABOUT 1600* F. AND THEREAFTER QUENCHING. 